The present device relates to a drop-out corrector circuit for video signals.
A drop-out corrector circuit is essential for detecting and correcting for drop-outs caused by dirt or damage to a recording medium in a playback device which reads a data signal from a recording medium such as a video disk.
An example of a conventional drop-out corrector circuit is shown in FIG. 1. In this drop-out corrector circuit, the RF (high-frequency) signal (FIG. 2A) forming the read-out signal from a recording disk is amplified by an RF amplifier 1 and supplied to an video demodulator 2 and to a drop-out detector 3. The video demodulator 2 is composed of an FM demodulator or the like, which outputs a color video signal extracted from the RF signal. This video signal is supplied to one of the terminals of switch 4 directly and to its other terminal via a 1H (horizontal synchronous period) delay circuit 5. In the drop-out detector 3, the amplitude of the RF signal is limited by a limiter amp 6, thus converting the signal to a pulse train signal, as shown in FIG. 2B. This pulse train signal is multiplied with the RF signal by a multiplier 7, and as the result is output as an RF signal with envelope elements, as shown in FIG. 2C, to a comparator 8. A reference signal is also supplied to the comparator 8. If the level of the signal with envelope elements is lower than the reference level, the output level of the comparator 8 immediately drops from a high to a low level. If the level of the envelope signal is higher than the reference level, the output level of the comparator 8 gradually changes with a specified time constant from a low to a high level. The output signal of the comparator 8 (FIG. 2D) is fed to another comparator 9 where it is compared with the reference level of the comparator 9. The low level output of comparator 9 (FIG. 2E) is fed to the activating terminal of the switch 4 as a drop-out detector pulse.
In the absence of a drop-out detector pulse, the switch 4 outputs the video signal from the video demodulator 2, but when a drop-out detector pulse is present, it outputs the 1H delayed video signal from the delay circuit 5.
With such a conventional drop-out corrector circuit, the reference level of the comparator 8 is preset at a fixed level. To improve the sensitivity of the drop-out detector in order to handle brief drop-outs with little fluctuation, it is sufficient to set the reference level of the comparator 8 at a higher level. However, if the sensitivity is raised too much, the comparator will pick up other elements in the RF signal as well as drop-outs, with the result that repeated switching of the video signal occurs at the switch 4, and hence the video signal is over-corrected, leading to the adverse effects such as flicker and distortion of the image of the screen. This is particularly evident with brighter images. Nonetheless, it is necessary to raise the sensitivity to the level of over-correction in order to detect brief drop-outs, since if the sensitivity is too low, the inadequate correction for brief drop-outs becomes a problem, especially with darker images.